Turning off the switch in medulloblastoma. The inhibitory acetylation of an oncogene

[No abstract available

MacroH2A1 isoforms are associated with epigenetic markers for activation of lipogenic genes in fat-induced steatosis.

The importance of epigenetic changes in the development of hepatic steatosis is largely unknown. The histone variant macroH2A1 under alternative splicing gives rise to macroH2A1.1 and macroH2A1.2. In this study, we show that the macroH2A1 isoforms play an important role in the regulation of lipid accumulation in hepatocytes. Hepatoma cell line and immortalized human hepatocytes transiently transfected or knocked down with macroH2A1 isoforms were used as in vitro model of fat-induced steatosis. Gene expressions were analyzed by quantitative PCR array and Western blot. Chromatin immunoprecipitation analysis was performed to check the association of histone H3 lysine 27 trimethylation (H3K27me3) and histone H3 lysine 4 trimethylation (H3K4me3) with the promoter of lipogenic genes. Livers from knockout mice that are resistant to lipid deposition despite a high-fat diet were used for histopathology. We found that macroH2A1.2 is regulated by fat uptake and that its overexpression caused an increase in lipid uptake, triglycerides, and lipogenic genes compared with macroH2A1.1. This suggests that macroH2A1.2 is important for lipid uptake, whereas macroH2A1.1 was found to be protective. The result was supported by a high positivity for macroH2A1.1 in knockout mice for genes targeted by macroH2A1 (Atp5a1 and Fam73b), that under a high-fat diet presented minimal lipidosis. Moreover, macroH2A1 isoforms differentially regulate the expression of lipogenic genes by modulating the association of the active (H3K4me3) and repressive (H3K27me3) histone marks on their promoters. This study underlines the importance of the replacement of noncanonical histones in the regulation of genes involved in lipid metabolism in the progression of steatosis

Immunopositivity for Histone MacroH2A1 Isoforms Marks Steatosis-Associated Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide. Prevention and risk reduction are important and the identification of specific biomarkers for early diagnosis of HCC represents an active field of research. Increasing evidence indicates that fat accumulation in the liver, defined as hepatosteatosis, is an independent and strong risk factor for developing an HCC. MacroH2A1, a histone protein generally associated with the repressed regions of chromosomes, is involved in hepatic lipid metabolism and is present in two alternative spliced isoforms, macroH2A1.1 and macroH2A1.2. These isoforms have been shown to predict lung and colon cancer recurrence but to our knowledge, their role in fatty-liver associated HCC has not been investigated previously

Protected from the inside: Endogenous histone deacetylase inhibitors and the road to cancer

Histone deacetylases (HDACs) play a crucial role in several physiological and pathological cell functions, including cell development and cancer, by deacetylating both histones and others proteins. HDACs belong to a large family of enzymes including Class I, II and IV as well as Class III or sirtuins subfamilies, that undergo a complex transcriptional and post-translational regulation. In current years, antitumor therapy is attempting to exploit several chemical classes of inhibitors that target HDACs, frequently reported to be misregulated in cancer. Nevertheless, the identity of gene products directly involved in tumorigenesis and preventing HDAC misregulation in cancer is still poorly understood. Recent evidence has demonstrated that the tumor suppressors HIC1 and DBC1 induce direct repression of Sirt1 function, whereas Chfr and REN(KCTD11/KAsH family) downregulate HDAC1, by inducing its ubiquitin-dependent degradation. Loss of these gene products leads to imbalanced enhancement of HDAC activity and subsequently to oncogenesis. All these genes are frequently deleted or silenced in human cancers, highlighting the role of endogenous HDAC inhibitors to counteracts HDAC-mediated tumorigenesis. Thus, endogenous HDAC inhibitors represent a promising class of "antitumor agents" thanks to which oncogenic addiction pathways may be selectively therapeutically targeted. (C) 2011 Elsevier B.V. All rights reserved

Multiple ubiquitin-dependent processing pathways regulate hedgehog/gli signaling: implications for cell development and tumorigenesis.

Hedgehog pathway is crucial for the maintenance and self-renewal of neural stem cells and for tumorigenesis. Hedgehog signaling is limited by multiple E3 ubiquitin ligases that process the downstream transcription factors Gli. Cullin family-based ubiquitination results in either Cullin1-Slimb/betaTrCP- or Cullin3-HIB/Roadkill/SPOP-dependent proteolytic processing or degradation of Drosophila Cubitus interruptus or mammalian Gli proteins. We have recently identified Itch as an additional HECT family E3 ligase, able to ubiquitinate and degrade Gli1. A functional link with the influence of Hedgehog signaling on cell development and tumorigenesis is suggested by the identification of Numb as a promoter of such an Itch-dependent ubiquitination process that leads to Gli1 degradation, thus suppressing its transcriptional function. Numb is an evolutionary conserved developmental protein that, during progenitor division, asymmetrically segregates to daughter cells thereby determining distinct binary cell fates. Numb is downregulated in cerebellar progenitors and their malignant derivatives (i.e. medulloblastoma cells). Furthermore, Numb has anti-proliferative and pro-differentiation effects on both cerebellar progenitors and medulloblastoma cells, due to its suppression of functional Gli1. These findings unveil a novel Numb/Itch-dependent regulatory loop that limits the extent and duration of Hedgehog signaling during neural progenitor differentiation. Its subversion emerges as a relevant event in brain tumorigenesis

Numb activates the E3 ligase Itch to control Gli1 function through a novel degradation signal

Hedgehog pathway regulates tissue patterning and cell proliferation. Gli1 transcription factor is the major effector of Hedgehog signaling and its deregulation is often associated to medulloblastoma formation. Proteolytic processes represent a critical mechanism by which this pathway is turned off. Here, we characterize the regulation of an ubiquitin-mediated mechanism of Gli1 degradation, promoted by the coordinated action of the E3 ligase Itch and the adaptor protein Numb. We show that Numb activates the catalytic activity of Itch, releasing it from an inhibitory intramolecular interaction between its homologous to E6-AP C-terminus and WW domains. The consequent activation of Itch, together with the recruitment of Gli1 through direct binding with Numb, allows Gli1 to enter into the complex, resulting in Gli1 ubiquitination and degradation. This process is mediated by a novel Itch-dependent degron, composed of a combination of two PPXYs and a phospho-serine/proline motifs, localized in Gli1 C-terminal region, indicating the role of two different WW docking sites in Gli1 ubiquitination. Remarkably, Gli1 protein mutated in these modules is no longer regulated by Itch and Numb, and determines enhanced Gli1-dependent medulloblastoma growth, migration and invasion abilities, as well as in vitro transforming activity. Our data reveal a novel mechanism of regulation of Gli1 stability and function, which influences Hedgehog/Gli1 oncogenic potential. Oncogene (2011) 30, 65-76; doi: 10.1038/onc.2010.394; published online 6 September 201

Molecular organization of the cullin E3 ligase adaptor KCTD11

The family of human proteins containing a potassium channel tetramerization domain (KCTD) includes 21 members whose function is largely unknown. Recent reports have however suggested that these proteins are implicated in very important biological processes. KCTD11/REN, the best-characterized member of the family to date, plays a crucial role in the ubiquitination of HDAC1 by acting, in complex with Cullin3, as an E3 ubiquitin ligase. By combining bioinformatics and mutagenesis analyses, here we show that the protein is expressed in two alternative variants: a short previously characterized form (sKCTD11) composed by 232 amino acids and a longer variant (IKCTD11) which contains an N-terminal extension of 39 residues. Interestingly, we demonstrate that IKCTD11 starts with a non-canonical AUU codon. Although both sKCTD11 and IKCTD11 bear a POZ/BTB domain in their N-terminal region, this domain is complete only in the long form. Indeed, sKCTD11 presents an incomplete POZ/BTB domain. Nonetheless, sKCTD11 is still able to bind Cul3, although to much lesser extent than IKCTD11, and to perform its biological activity. The heterologous expression of sKCTD11 and IKCTD11 and their individual domains in Escherichia coil yielded soluble products as fusion proteins only for the longer form. In contrast to the closely related KCTD5 which is pentameric, the characterization of both IKCTD11 and its POZ/BTB domain by gel filtration and light scattering indicates that the protein likely forms stable tetramers. In line with this result, experiments conducted in cells show that the active protein is not monomeric. Based on these findings, homology-based models were built for IKCTD11 BIB and for its complex with Cul3. These analyses indicate that a stable IKCTD11 BTB-Cul3 three-dimensional model with a 4:4 stoichiometry can be generated. Moreover, these models provide insights into the determinants of the tetramer stability and into the regions involved in IKCTD11-Cul3 recognition. (C) 2011 Elsevier Masson SAS. All rights reserved